A Z-θ table is described, for example, in U.S. Pat. No. 6,700,249, in which the Z-θ table includes two stacked circular motors, that is, arranged one above the other in the table's Z direction. The lower motor includes a fixed external stator and a central rotor occupying the interior space of the stator. The upper part of this central rotor is threaded. The upper motor also includes an external stator associated with a sliding device enabling this stator to undergo displacement along the vertical Z axis. This upper stator includes a base with an annular central part forming a nut associated with the threaded part of the rotor of the lower motor. Therefore, whenever the lower rotor is activated, the stator of the upper motor undergoes vertical displacement. The rotor of the upper motor is independent of the linear movement and allows angular positioning of the table. Linear movement is controlled by the single lower motor, whereas rotation is controlled only by the upper motor. To control both movements, the central lower rotor is associated with a circular encoder, while the upper motor stator is associated with a linear encoder mounted on the periphery of the table. Each of these two degrees of freedom, therefore, is associated with its own individual motor. In this sense, the two motors are independent, which represents a certain advantage in controlling the Z-θ table.
However, the table described in U.S. Pat. No. 6,700,249 is believed to present several drawbacks. Because of the arrangement of the two stacked motors, the table height is relatively significant. An angular offset of the central axis of the lower motor relative to the vertical axis engenders a positioning error that increases proportionally with the height of the table. The mass of the table is relatively high. Also, because the upper motor has a stator that undergoes vertical displacement, the power cable of this upper motor undergoes movement whenever the table is activated, which can be detrimental, particularly for clean-room applications. The ball-screw system between the two motors presents a relatively small diameter with respect to the dimensions of the table, which results in a limit to the load that can be carried by the table described above. This also reduces the stability of the table. Moreover, the load supported by the table is taken up by the fixed lower stator through two bearings or ball bearings, which is an impediment to the rigidity and stability of the table. In particular, there is a risk that a decentered load will tilt the Z-θ table base.